FIGS. 1(a) to 1(c) illustrate a typical 3-phase switched reluctance (SR) machine and common electronic switching circuits which may be used to control the machine. The machine essentially consists of a stator s defining salient stator poles 1, 1', 2, 2', 3, 3', on which are wound phase windings w, of which only one is shown in association with the set of poles 2,2', and a rotor r with salient poles 4, 4', and 5, 5'. The electronic switching circuits are arranged to supply unidirectional currents to the phase windings w.
In both control circuits, each phase winding in the machine of FIG. 1(a) is associated with a circuit leg comprising at least one electronic switch t in series with each winding w across a dc supply (+/-).
The known means for controlling a switched reluctance machine include three characteristic regions which can be designated `low`, `intermediate` and `high` speed regions. These will be explained below. (For background see also for example `Variable Speed Switched Reluctance Motors`, by Prof. P. J. Lawrenson et al, IEE Proceedings--B, Vol.127, No.4, July 1980).
At `low` speed the current is controlled by chopping. Two switching strategies are well known for chopping using circuits of the form of FIG. 1b.
In the first method, both switches t of a limb are switched together, so that at switch-off the current transfers from the switches to flow through the diodes d. In the second method, only one of the two switches t is opened, so that the current circulates, or `freewheels`, through one switch and one diode. Both switches are turned off at the end of a phase conduction period. Freewheeling is not possible with the circuit of FIG. 1c.
FIGS. 2(a) and (b) illustrate typical motoring and generating phase winding current waveforms, respectively, without freewheeling. The current is illustrated in relation to angle of rotation of the rotor with respect to the stator, (.theta.-theta).
Thus, at `intermediate` speeds, the angle of rotation required for the growth and decay of flux is significant in relation to the phase period (defined as the angle of one cycle of phase inductance variation). The time rate of change of flux linkage is determined by the voltage applied to the winding and therefore the rate of change with respect to angle falls as the speed rises. At `intermediate` speeds there is therefore only a single pulse of current in the switch(es) and diode(s) in each phase period. Corresponding phase winding currents are illustrated in FIGS. 3(a) and (b), respectively for motoring and generating operation. Operation in this manner is called the `single-pulse` mode of operation. It should be noted that the `conduction angle` (.theta..sub.c -theta.sub.c) in FIGS. 3(a) and (b) is the angle over which the switches are closed; .theta..sub.on -theta.sub.on is the `switch-on angle` and .theta..sub.off -theta.sub.off is the `switch-off angle`. Furthermore, the effect of the phase winding resistance has been assumed to be negligible in constructing the waveforms of FIGS. 3(a) and (b). The flux linkage waveform (u/-psi) of the phase winding is illustrated by the broken lines. Following closing of the switches t in FIG. 1b associated with a phase winding, the flux linkage grows linearly. When the switches are opened, the flux linkage falls linearly, the current flowing through the diodes d imposing a voltage of -V.sub.S on the windings. (The operation of the circuit of FIG. 1c is the same, except that, following the opening of the switch, voltage -V.sub.H is impressed on the winding).
In order to maintain the torque developed by the machine as the speed rises under `single-pulse` control, it is necessary to maintain the flux amplitude. This is commonly achieved by increasing the `conduction angle` with speed.
Control in this `single pulse` mode of operation with full torque capability, which may be related to a notional `intermediate` speed range, is exercised by variation of the angle of a rotor pole relative to a particular stator pole at which the switches are closed (the `switch-on angle`) and the angle over which the switches remain closed (the previously mentioned `conduction angle`).
It will be apparent to the skilled person that, under the idealised, resistanceless assumptions behind FIGS. 3(a) and (b), if the conduction angle is made greater than half the complete phase period, the flux and current will fail to return to zero at the end of one cycle. This will lead to progressive uncontrolled growth of flux and current with successive cycles. This is acknowledged by the person skilled in the art to be a highly unstable condition.
As far as the inventors are aware, it has always been considered by the skilled person that such a situation is undesirable, ie. a conduction angle corresponding to half the phase period has been taken to be the upper limit on machine control. Indeed, it has been stated in the paper `Optimal-efficiency excitation of variable-reluctance motor drives` by Prof. D. A. Torrey and Prof. J. H. Lang in IEE Proceedings-B, Vol. 138, No.1, January 1991 in discussing the optimum control strategy for a doubly salient pole variable-reluctance motor (here referred to as a switched reluctance motor) that, "Since .theta..sub.cond =180.degree. corresponds to a 50% duty cycle for the controllable switch, assuming no current chopping, a larger conduction period would result in the current building up in the phase from cycle to cycle, eventually being stopped by the current regulator, but rendering loss of control. This is clearly to be avoided."
It has been explained above that in order to maintain the torque as the speed rises, the `conduction angle` is commonly increased with speed. However, because of the above imposed upper limit on the `conduction angle` of half the phase period, the torque can only be maintained up to a certain speed. This is the upper limit of the `intermediate` speed range. Operation is possible at higher speeds, in the `high` speed range, but only at the expense of a decreasing torque capability because of the falling flux amplitude.
Thus, the operation of switched reluctance machines with conduction angles greater than half the phase period has been avoided in the known art, because this `continuous current` mode is associated with an unstable flux growth and consequent highly unstable operating characteristics. However, the inventors of the present invention have recognised that exploitation of the `continuous current` mode would enable the torque to be maintained in the `high` speed region, thus avoiding the decrease in torque with speed explained above, if it were possible to control the machine stably in this mode.